AU-G99X - DC Offset Left Channel Problem

Discussion in 'Exclusively Sansui' started by absolve2525, Nov 11, 2017.

  1. absolve2525

    absolve2525 AK Subscriber Subscriber

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    I've been following the tech manual to set bias and dc offset, but having a problem with the left channel DC offset. It keeps bouncing around, going as high as a couple hundred mv suddenly. Rapidly going up and down either direction +/-. The other channel is stable at 5mv. Bias is stable for both channels. Thanks for any suggestions!
     
  2. slimecity

    slimecity Super Member

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    Presumably you have replaced the DC offset trimpot (KVR2)?

    You have a uPA68H FET at the input stage of the power amp module. One symptom of not being able to set the DC offset properly could be a bung fet.

    I will let anyone with a bit more experience chime in here.
     
  3. absolve2525

    absolve2525 AK Subscriber Subscriber

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    Thanks for the reply. I haven't changed the trim pot yet. The other trim pots were fine, albeit somewhat tricky/sensitive. Are there modern replacements available for that FET? If that's the problem, of course.
     
  4. Robisme

    Robisme Sansui Enthusiast Subscriber

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    How is the DC voltage coming from the preamp?

    Rob
     
  5. absolve2525

    absolve2525 AK Subscriber Subscriber

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    I'm not sure! Where do I measure that? Thanks
     
  6. Karl vd Berg

    Karl vd Berg Super Member

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    Yes, the LSK489, ultra-low-noise, dual j-FET
     
  7. absolve2525

    absolve2525 AK Subscriber Subscriber

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    Where can I measure voltage from the preamp? Thanks
     
  8. slimecity

    slimecity Super Member

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    It wont be easy as you dont have "pre out" RCA sockets on this - and Im not sure if its entirely necessary. Best bet IMO is to replace the input FETs to check if the problem persists after doing this. But thats just my opinion.
     
  9. petehall347

    petehall347 the brandy coffee man

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    trimpot change is a good idea ..order fets on standby just in case
     
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  10. absolve2525

    absolve2525 AK Subscriber Subscriber

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  11. slimecity

    slimecity Super Member

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    Yup thats them - based on Karl vd Bergs' recommendation above.

    Get em while you can - replacing FETs is a real PITA in vintage gear - a good replacement for obselete FETs isnt usually around for long......
     
  12. absolve2525

    absolve2525 AK Subscriber Subscriber

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    I ordered the FETs! ☺️ I need 470 ohm pots for the DC offset and 100 ohm for the hot/cold balance. Do you guys have a link or recommendation for these? Thanks! I look forward to getting this bad boy running properly.
     
  13. Karl vd Berg

    Karl vd Berg Super Member

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    Hi, you ordered the SOT-23 package FETs, they have a different pinout, like the other TO-71 package. I should have mentioned it, too. But probably you will need them anyway.

    The link for the other TO-71 (6L) package is this: https://www.trendsetter.com/Transistor-p/lsk489 to-71 6l.htm

    [​IMG]

    The stock FETs in your AU-G99X are the uPA68H's, which are 7-SIP package and have this pinout below:

    [​IMG]

    You need to adapt the SOT-23 package pinout to the original uPA68H one, by using a small perfboard.

    [​IMG]

    [​IMG]

    I know it's not the easiest way, but considering that good quality FETs are really scarse, your nice AU-G99X deserve another chance...
     
    Last edited: Nov 16, 2017
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  14. absolve2525

    absolve2525 AK Subscriber Subscriber

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    OK, which do I order that don't require an adapter? The link in your above message? Thanks
     
  15. Hyperion

    Hyperion Roobarb & Custard Subscriber

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    The 7 pin SIP package for the original device is obsolete, so your choices are SOT-23, SOIC-A or TO-71 style packages, there is no currently available package for that device which will not require an adapter.

    I think you will need to make an adapter as Karl has suggested.
     
    Last edited: Nov 17, 2017
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  16. absolve2525

    absolve2525 AK Subscriber Subscriber

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    Thanks, I can try to make an adapter, but I'd need more details/pictures of how to align the leads and where go to get the boards. If there's already a thread on adapters, I can try s search. What do you use for the leads? Thanks again.
     
  17. absolve2525

    absolve2525 AK Subscriber Subscriber

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    Anybody made adapters?
     
  18. petehall347

    petehall347 the brandy coffee man

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    sorry but cant concentrate with raging toothache .. but check to see if this will all line up properly .
    [​IMG]
     
  19. absolve2525

    absolve2525 AK Subscriber Subscriber

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    Thanks for the reply! Is that adapter available to purchase anywhere?
     
  20. petehall347

    petehall347 the brandy coffee man

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    search google for SOT-23 adaptor .
     

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